Audio systems utilizing amplifier circuits for amplifying audio source signals in order to drive audio devices have been widely used in various electronic products. In a typical application circuit, as shown in FIG. 1, an audio amplifier chip 100 is used to drive an earphone 104 or a loudspeaker 106 based on an audio source signal 102. The audio source signal 102 is fed into the chip 100 through a coupling capacitor 108 and an input resistor 110, and the chip 100 includes two operational amplifiers 112 and 114, with the inverting and non-inverting inputs 116 and 118 of the operational amplifier 112 connected to the input resistor 110 and a reference signal Vref, respectively, so as to generate an output signal Vo+ at the output 120 of the operational amplifier 112 to drive the earphone 104. The output 120 of the operational amplifier 112 is connected to the other operational amplifier 114 by a resistor 112. To generate the reference signal Vref, a voltage divider composed of two resistors 132 and 134 are connected to a supply voltage VDD by a power input 136, in such a way that the reference signal Vref of VDD/2, for example, is generated at a node 138, to connect to the non-inverting inputs 118 and 128 of the operational amplifiers 112 and 114, respectively. A capacitor 140 may be coupled to the power input 136 to stable the supply voltage VDD for the chip 100. A bypass capacitor 142 is coupled to the node 138 to be charged to the voltage VDD/2, and the capacitance CB of the bypass capacitor 142 determines the charge rate of the reference signal Vref up to VDD/2. The operational amplifier 112, the input resistor 110, and a feedback resistor 144 connected between the output 120 and the inverting input 116 of the operational amplifier 112 constitute a well-known amplifier, and in which the feedback resistor 144 and the input resistor 110 are both outside of the chip 100 for adjusting the gain of the amplifier. However, the feedback resistor 144 and the input resistor 110 may be integrated in the chip 100 instead. The other amplifier is constituted by the operational amplifier 114, the input resistor 122 and a feedback resistor 114, and has a unit gain, for inverting the output signal Vo+ to generate another output signal Vo−. The output signals Vo+ and Vo− constitute a pair of differential output signals for driving the loudspeaker 106. The chip 100 further comprises a bias control circuit 146 to generate two control signals 152 and 154, based on a sleep or shutdown signal SHUTDOWN originated from the system control unit and a select signal or bridge-tied-load signal SEL/ BTL originated from the earphone socket, for the control of enabling and disabling the operational amplifiers 112 and 114. When powers on, owing to the slew rate of the operational amplifier 112 different from the rate that the reference signal Vref is charged up to a predetermined level, e.g., VDD/2, an instant variation occurs in the signal at the output 120 of the chip 100, resulting in an unfavorable noise, referred to as ‘pop’, emanated from the earphone 104 or loudspeaker 106.
Accordingly, various arts have been proposed for suppression of such power-on pops. Soft start-up is employed in U.S. Pat. No. 5,796,303 issued to Vinn et al. to prevent the operational amplifier from imparting an output voltage to the loudspeaker, together with pre-charging the output of the amplifier circuit up to a predetermined voltage, in such a manner that the power-on pop is suppressed. The output of the amplifier circuit is biased when powers on by U.S. Pat. No. 6,040,740 issued to Dondale, so as to suppress the power-on pops. Hellums provides two different bias currents in U.S. Pat. No. 6,316,993 for the operational amplifier under the control of a mode control signal, such that pops are reduced during power-on period. On the other hand, in U.S. Pat. Nos. 5,642,074 and 5,703,529 issued to Ghaffaripour et al., and U.S. Pat. No. 5,939,938 issued to Kalb et al., the feedback resistor of the amplifier is first bypassed during power-on period, and then restored once the reference signal of the amplifier is charged up to a predetermined voltage, so as to switch the gain of the amplifier to reduce the pops. The control signal for restoring the feedback resistor is further delayed in U.S. Pat. No. 6,346,854 issued to Heithoff, for a better pops reduction. For more illustrative, the scheme employed in these arts are shown by FIG. 2, in which a switch 200 is inserted between a voltage-dividing resistor 133 and the supply voltage VDD, and an amplifier 202 has an inverting input 204 connected with the reference signal Vref, a non-inverting input 206 connected with the predetermined voltage supplied by a voltage divider composed of two resistors 210 and 212, and an output 208 to provide a control signal that is further delayed by a delay circuit 216 to generate a delayed control signal 218 to switch a switch 220 shunt to the feedback resistor 144. By switching the switch 220, the feedback resistor 144 is removed or restored to switch the gain of the amplifier, and therefore the power related pops are reduced.
In the above conventional arts, however, there are still imperfections to be improved. Therefore, it is desired a further de-pop improvement to the amplifier circuit.